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[release/3.0] Updating Math.Round and MathF.Round to be IEEE compliant so that the intrinsic and managed form are deterministic.#26017
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…intrinsic and managed form are deterministic. (#25901) * Updating Math.Round and MathF.Round to be IEEE compliant so that the intrinsic and managed form are deterministic. * Fixing the Math.Round and MathF.Round handling for values greater than 0.5 and less than 1.0 * Applying formatting patch.
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This is a backport of #25901 |
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CC. @CarolEidt, @AndyAyersMS, @jkotas, @danmosemsft I added one additional commit, which addresses @CarolEidt's request that we document that only the |
AndyAyersMS
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Aug 5, 2019
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Bug exposed by enabling tiering by default in 3.0 |
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stephentoub
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Branch is open, merging |
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Description
The managed implementations of the Math.Round functions were not IEEE compliant and would return a different result between Debug and Release mode. In .NET Core 3 with the tiered JIT support they now return a different result for the first 29 calls of the function before the JIT recognizes them as "hot" and they are rejitted.
Customer Impact
Customers calling
Math.RoundorMathF.Roundmay get different behavior between two consecutive calls to the functions.Regression?
Yes, as per the description.
Risk
Low. This is pulling in a port of a known good implementation from the BSD SoftFloat library. It was additionally validated that all 4 billion inputs for the single-precision input range are returning the correct values as compared to the intrinsic forms of these APIs (which compile down to the
roundssandroundsdinstructions).